Radio Frequency Identification (RFID) transponders (tags) are generally used in conjunction with an RFID base station, typically in applications such as inventory control, security, access cards, and personal identification. The base station transmits a carrier signal that powers circuitry in the RFID tag when it is brought within a read range of the base station. Data communication between the tag and the station is achieved by modulating the amplitude of the carrier signal with a binary data pattern, usually amplitude shift keying. To that end, RFID tags are typically integrated circuits that include, among other components, antenna elements for coupling the radiated field, rectifiers to convert the AC carrier signal to DC power, and demodulators to extract the data pattern from the envelope of the carrier signal.
If fabricated at sufficiently low cost, RFID tags can also be useful in cost-sensitive applications such as product pricing, baggage tracking, parcel tracking, asset identification, authentication of paper money, and animal identification, to mention just a few examples. RFID tags could provide significant advantages over systems conventionally used for such applications, such as bar code identification systems. For example, a basket full of items marked with RFID tags could be read rapidly without having to handle each item, whereas they would have to be handled individually when using a bar code system. Other advantages of RFID tags over bar codes include higher read speed, less susceptibility to problems such as dirt obscuring a portion of the code, no requirement of exact alignment with the label, and no requirement of line of sight. Unlike bar codes, RFID tags provide the ability to update information on the tag. Nevertheless, conventional RFID technology is too expensive for dominant use in such applications.
RFID tags may be active or passive. While active RFID tags contain their own power source, passive RFID tags obtain their power from an RF field radiated by the base station. Passive RFID tags are substantially less expensive than active RFID tags, making passive RFID tags a good choice for low cost applications. However, passive field powered RFID tags require at least an order of magnitude more power in the interrogation signal from the base station than an active RFID tag.
Even conventional passive RFID tags are too expensive for dominant use in applications dominated by bar codes today. A major factor driving up fabrication costs of RFID tags is the size of the silicon integrated circuit that makes up the tag. Passive RFID tags generally have an external antenna element for coupling the radiated field. Conventionally, the external antenna element is physically bonded to the RFID tag. Conventional RFID tags require at least two pads large enough to bond wire for the attachment of the external antenna coil. Since RFID tag chips are generally relatively small compared to the size of a bond pad, these bond pads consume a significant percentage of the integrated circuit area of a conventional RFID tag.
One prior solution for reducing the number of bond pads on a passive RFID tag chip includes a thin film antenna included on the integrated circuit. This method eliminates the need for the bonding pads, thereby decreasing the silicon surface area and consequently the fabrication cost. Eliminating the wire bonding process further reduces fabrication costs. However, since such integrated antennas are form-factor constrained and are necessarily small, the coupling efficiency to the radiated field substantially reduced in RFID tags with an internal antenna. Consequently, this method often results in exceedingly short operating read distances from the base station.
Increasing the transmitted power of the field radiated by the base station can increase the operating read distance. However, maximum levels permissible are limited by government regulation.